The present invention relates generally to the field of geophysical exploration and more particularly to a method of magnetotelluric exploration employing measures of the earth's magnetic and electric fields.
The magnetotelluric method makes use of the propagation properties of electromagnetic waves in conducting media so as to obtain a measure of the earth's resistivity or conductivity as a function of depth. Since the magnetotelluric method employs naturally occurring magnetic and electric fields, it is referred to as a passive electrical method of exploration in contrast to active exploration methods wherein an electric generator is used to induce a signal. The earth's electromagnetic fields cause a flow of telluric current in the earth's crust which depends upon the conductivity or resistivity of the earth's geologic formations. If the conductivity or resistivity is calculated and displayed, geophysicists can infer information about the earth's geologic structure. This technique is particularly useful in areas where other geophysical survey methods are inadequate.
A first method of magnetotelluric exploration was described by Cagniard in "Basic Theory of the Magnetotelluric Method of Geophysical Exploration" Geophysics vol. 18 p. 605 (1953). This method consists of simultaneously measuring the variations in one horizontal component of the earth's electric field and an orthogonal component of the earth's magnetic field over an extended period of time. These measurements can then be converted into frequency components by means of a Fourier transform. The ratio of the frequency component of the electric field to that of the magnetic field is a wave impedance that is a function of frequency. Since the depth of penetration of an electromagnetic wave into the earth's formations is related to the square root of the earth's resistivity divided by the frequency of the electromagnetic wave and the conductivity of the earth's formation, the wave impedance can be used to estimate the conductivity or resistivity distribution in the earth's formations.
Cagniard made his estimate of the earth's resistivity distribution using a mathematical model in which the earth's resistivity varied only with depth, i.e., a one-dimensional model. Cagniard's method was subsequently refined by others to be used with both two- and three-dimensional models of the earth's conductivity or resistivity structure. The history of advances in magnetotelluric exploration has been one of a gradual appreciation of the complexity of the earth's geological structure triggering a search for new methods of collecting magnetotelluric data as well as new methods for deciphering such data so as to reliably interpret such complex structures. Until now, the magnetotelluric data collected within a region of interest have always been a subset of the complete electric and magnetic fields required to fully describe their spatial variations caused by complex geologic structures of the earth.
Typically, magnetotelluric exploration comprises laying out one or more discrete sensing sites at which orthogonal, horizontal components of the earth's electric field (e.g., E.sub.x, E.sub.y) and magnetic field (e.g., H.sub.x, H.sub.y) can be measured. It has also been found advantageous to also record the vertical component of the magnetic field (H.sub.z). The sensing sites are considered discrete because the electric field measured at one sensing site is noncontinuous with the electric field measured at another sensing site. More simply, the electric field measurements obtained at one sensing site are spatially independent of the electric field measurements obtained at another sensing site.
As magnetotelluric exploration is currently practiced, an impedance tensor (Z.sub.ij) is calculated for selected frequencies of the measured electric and magnetic fields obtained at each discrete sensing site. Each impedance tensor (Z.sub.ij) can then be processed so as to effect a rotation of the coordinate axes along which the electric and magnetic fields were measured in an attempt to minimize the principal diagonal elements of the impedance tensor (i.e., Z.sub.xx and Z.sub.yy). For resistivity structures which are truly two-dimensional, the principal diagonal elements (Z.sub.xx and Z.sub.yy) must be zero. Consequently, the magnetotelluric data can then be processed as if there existed a horizontal direction along which the earth's resistivity is assumed constant. By convention, such direction of constant resistivity is referred to as the strike direction. The components of the electric and magnetic fields can thus be separated into elements parallel and perpendicular to the strike direction.
Prior art magnetotelluric methods work well when the area being surveyed has a resistivity distribution which varies only in one or two dimensions. Unfortunately, such one- and two-dimensional variations represent only a small minority of the actual resistivity distributions in the earth. When magnetotelluric measurements are made over a structure having other than a simple one-or two-dimensional resistivity distribution (e.g., three-dimensional), the following problems can be encountered. First, it is generally not possible to identify an acquisition coordinate system based on strike direction. Hence, whatever acquisition coordinate system is used, it is not always possible to separate the electric field into components either parallel or perpendicular to the strike direction. Moreover, the main disadvantage of conventional magnetotelluric methods is that they can give unreliable results when the conductivity of the earth's subsurface formations vary in all three dimensions because the sensing sites are isolated one from another such that the measurements obtained at one sensing site are independent of measures obtained at another sensing site. Prior attempts to overcome this unreliability have involved large computational and human efforts.
More recently, Bostick described in "Electromagnetic Array Profiling," 50th Annual Meeting Society of Exploration Geophysicists, page 60 (1986) a method of electromagnetic surveying which can give more reliable results in the presence of certain forms of three-dimensional variations in the earth's resistivity the so-called "statics" effect. Bostick's electromagnetic array profiling (EMAP) method consists of measuring variations in the earth's magnetic field along two horizontal nonparallel directions at one point in an area to be surveyed and simultaneously measuring one component of the earth's electric field at a plurality of sensing sites along a generally linear survey line. Additionally, the electric field measurements made at the plurality of sensing sites are not spatially independent of one another and thus can be said to be continuous.
While Bostick's method is an improvement with regards to solving the statics problem, it represents a step backwards since the resultant of such a method is a scalar without the directional information contained in the impedance tensor of traditional magnetotelluric exploration.
As a consequence of measuring only one electric dipole component of the earth's electric field along the line of profile, the EMAP technique cannot measure the complete impedance tensor along the line of profile as with the conventional magnetotelluric method of exploration nor can the EMAP method determine the strike direction of the earth's formations. Additionally, the EMAP technique cannot truly generate two-dimensional estimates of the earth's resistivity structure. Rather, the EMAP technique generates a continuous series of one-dimensional estimates of the earth's resistivity structure along the line of profile so as to emulate a two-dimensional profile of the earth's substructure. Thus, the EMAP technique barters reduced acquisition and collection costs for reduced information about the earth's resistivity structure.
In spite of the advances in magnetotelluric exploration, present methods of magnetotelluric data acquisition and processing remain unreliable for three-dimensional variations in the earth's resistivity distribution. In view of such shortcoming, the present invention provides a novel method of magnetotelluric exploration for collecting and processing magnetotelluric data so as to provide more reliable estimates of the earth's resistivity structure, especially when the earth's resistivity structure is not simply one- or two-dimensional.